The Mona Island iguana is endemic to the remote island of Mona, a low-profile limestone plateau situated midway between Puerto Rico and Hispaniola. The 11 by 7 km island lies within a deep sea channel known as the Mona Passage, and present submarine banks offer no evidence of former connections with either Puerto Rico or Hispaniola. The entire island is occupied by iguanas.

Census figures indicate that the Mona iguana population is abnormally small. A survey of the similarly-sized rhinoceros iguana on Petite Gonave Island in Haiti indicated densities 26 times greater than those found on Mona. Wiewandt found that immature iguanas were scarce on Mona, representing only 5-10% of the population (Weiwandt 1977), and Moreno sighted only two juveniles among 118 iguanas seen (Moreno 1995). This contrasts sharply with Garcia’s unpublished data for rhinoceros iguanas (C. cornuta cornuta) on Isla Beata in the Dominican Republic, where all age classes are abundant and juveniles comprise approximately one-third of the population (see Grupo Jaragua 1994). Low iguana densities and the scarcity of juveniles on Mona suggest a senescent and declining population.

Mona’s climate is dry subtropical (800 mm rainfall per year), supporting an open canopy forest of short, seasonally deciduous trees, shrubs, cacti, and bromeliads. Rainwater percolates rapidly through the porous limestone substrate, allowing no freshwater streams or ponds. Solution channels and sinkholes that penetrate the island’s rock topography offer underground shelters that are utilized by both male and female iguanas, and retreats attractive to females are vigorously defended by males. Some males hold territories year-round, while others defend them only during the brief June mating season.

Because more than 95% of Mona’s surface is rock, females must migrate to scarce soil deposits for nesting. The onset of the two-week, mid-summer nesting season appears to be cued by photoperiod and females are especially wary at this time. Most egg laying (74%) occurs in sandy clearings on the island’s southern coastal terraces, with the remaining 26% in sinkhole depressions (Haneke 1995). Mean clutch size is 12 eggs. Surviving eggs hatch approximately 83 days after laying, during the latter half of October. Newly emerged young are large and only the smallest juveniles are susceptible to indigenous predators. Coloration and behavior of hatchlings suggests that aerial predators have long been a threat to this age class (Wiewandt 1977).

Females require 6 to 7 years to reach sexual maturity. Although longevity records are not available, Mona iguanas, like all large rock iguanas, are probably among the longest lived lizards in the world. Consequently, populations are slow to recover from losses over time.

Mona iguanas are primarily herbivorous, with a strong preference for fruits that fall from native trees. Some animal matter is eagerly taken, especially caterpillars when available. Trees reach their greatest size and diversity in scattered sinkhole depressions, areas that are of particular importance to the welfare of the iguana population.

The most pressing conservation management challenge on Mona today is that of exotic species. Having evolved in the near absence of predators, insular iguanas lack the behavioral and demographic attributes to cope with introduced mammals. Feral pigs regularly plunder iguana nests.

Feral cats are also present on Mona, and constitute the most serious threat currently impacting young iguanas. The devastating impact of cats on a population of Turks and Caicos iguanas was clearly documented (Iverson 1978), and there is little doubt that the present scarcity of juveniles on Mona is due primarily to the combined effects of pigs consuming eggs and cats preying on young.

Habitat modification due to intense browsing pressure by feral goats (resulting in forest trees being unable to propagate successfully and a severe reduction in leaf litter) may also threaten the iguanas.

Although lacking permanent settlements, Mona is a haven for recreational activities, including camping, fishing, swimming, scuba diving, beach combing, exploring, and hunting. Most of these activities are concentrated along the island’s sandy coastal terraces and within sinkhole depressions, areas of critical importance for iguana nesting. Haneke (1995) observed that new camping facilities had been recently added in iguana nesting areas at Playa de Pajaros. Mona iguanas are wary and easily disturbed while nesting, and visitors can unintentionally disrupt the egg-laying process. People and feral animals walking through nest site clearings during incubation may cause nest chambers to capsize, denying oxygen to developing eggs. These and other conflicts between iguanas and visitors are bound to intensify as recreational use of the island continues to expand. Goats regularly gather in sinkhole depressions on Mona’s plateau, and this may partially explain Heneke’s (1995) observation of complete nest failure there. Mona may have already exceeded its carrying capacity for low impact tourist visitation. Better supervision over visitors, particularly by strengthening educational programs, will become increasingly important as the number of people coming to Mona continues to grow.

Another recent concern for conservation of Mona iguanas is the emergence of an undefined disease or parasite that causes blindness. Recently, Haneke (1995) observed 15 blind adults on Mona, all with opaque, bluish eyes and apparently severely undernourished. Ramos (1964; cited in Kuns et al. 1965) lists 16 species of eye flies (family Chloropidae) occurring on Mona, including Hippelates pusio, which has been incriminated in the spread of catarrhal conjunctivitis in the United States. Studies are urgently needed to identify the pathogens and vectors responsible for blindness in the Mona iguana population.

The Mona iguana is listed as threatened by the U.S. Fish and Wildlife Service and the PR-DNRE. In 1984, the U.S. Fish and Wildlife Service approved a recovery plan for the Mona iguana prepared by C. Diaz, PR-DNRE (Diaz 1984).

During the last 25 years, the PR-DNRE has instituted some important changes. The hunting season on Mona has been moved to a time outside the iguana nesting and incubation seasons. Together with the local herpetological society, Sociedad Chelonia, the government has created several new nesting areas on the southwestern coastal terrace. A number of clearings in the Casuarina forest have been established that are fenced off from goats and pigs but allow iguanas to pass freely. Fencing of remote nest sites is currently being undertaken by the PR-DNRE, the U.S. Fish and Wildlife Service Caribbean Office, the U.S. Coast Guard, the Sociedad Chelonia, and the Toledo Zoo. Researchers at the PR-DRNE and the Toledo Zoo have additionally begun to assess the nature of the blindness syndrome seen in several adult iguanas. Currently, PR-DNRE is conducting a long-term study to quantify the impact of feral cats on Mona Island wildlife.